Van Moisture Situation

Some thoughts on the moisture situation in camper vans.

People generate moisture through breathing and perspiration — from about 40 grams per hour sleeping up to 90 grams per hour doing light work.  So, two people might add 3000 grams of water to the van in a full day/night.  A dog would add some more.  Cooking or even making coffee or tea contributes moisture, this can be up to 3000 grams per day, but likely less in most van settings.  Other sources include showers (if so equipped) and drying out wet cloths.  Non-vented propane heaters also add moisture if used.

So, how much moisture does it take to get the van relative humidity up to (say) 70%?
A typical van volume is about 630 cubic ft, which is 47 lbs of air.  The water weight with 70F temperature and 70% relative humidity is 5 grams per lb of air.  So, the total water weight in the van for 70F and 70%RH is about 240 grams.

Vans have a metal outer skin that is a near perfect vapor barrier, so none of the moisture gets out via that route.

So, it seems like with people etc. generating several thousand grams of moisture per day and it only taking 240 grams of water weight to get the van up to 70% humidity, that: 1) there needs to be good ventilation in the van to get rid of the excess water, and 2) that even with good ventilation there are likely to be extended periods when the relative humidity in the van will be high.

In this test, all four insulation samples are tested at 50%, 60% and 70% relative humidity.  And the 60% and 70% tests are run for two full days.

This makes the van a challenging environment for moisture management.


The setup consists of a box that is about  4ft high by 5 ft wide by 3 ft deep. The front face of the box is glazed with 1/8 inch plexiglass and the rest of box is constructed with rigid insulation board.  The Plexiglass[GR1]  is used in place of the van sheet metal so that any condensation can be directly observed. The plexiglass face is divided into four bays, each of which can accommodate an insulation sample.  In addition, there is a Plexiglass window on the side of the enclosure so that condensation level on a “window” can be compared to condensation for the insulation samples.

The box has a heater (light bulbs) and a humidifier which are run by a controller so that humidity and temperature inside the box can be controlled.  The box is located in my garage/shop and the temperature is roughly regulated by opening doors to cool or turning on the heater if it gets too cold.  The entire back of the chamber is easily removable, so that during the daytime drying periods the back can be partially opened to provide more ventilation for drying.

<pics  of chamber> The Polyiso panel (right) is sealed around the edges with Great Stuff urethane foam to try to keep water vapor from migrating to Plexiglass side of the panel.  The center part of the Polyiso panel is removable so that it can be weighed for moisture pickup during the test.